idioms doesn't make sense for the .io extension.
idiom.io is a different story, because the word is singular. Singular words lead themselves open to interpretation and in most cases, can become brands. These kinds of "open vessel" names work well with .io.
As soon as you pluralize it, then you site kind of haves to be about idioms. Can someone come along and build a site about idioms on this domain? Sure. But is it likely? Probably not. Could it possibly be used as a name-brand? It's possible, because that's what idioms.com is doing.
But as far as investment is concerned, domains with "idiom" in it aren't flying off the shelves.
4 reported sales in all of history.
This is the kind of name that will sit on the shelve for a long time and will bleed you dry. But you won't know, so put up a For Sale page with Dan.com or Efty and see how much traffic and offers the name actually gets then make a decision from there.